Liquid crystals (LCD's) are commonly employed to display alphanumeric data obtained by electronic instrumentation in response to measured parameters, such as voltage, current, resistance, etc. A liquid crystal material is one which is birefringent and exhibits interference patterns and polarized light resulting from parallel orientation of large clusters of molecules. Depending upon the magnitude of voltage applied between parallel back plane and segment conductors, characters are formed by light reflected from the particular segments upon which the display voltage is applied. Because LCD devices have a very high internal impedance, the current drawn by the devices is small marking LCD's desirable for battery-operated instrumentation.
In compact instrumentation, the LCD is typically mounted on a printed circuit board having a conductive pattern for applying electrical signals to corresponding conductors on a surface of the LCD extending from the segment electrodes. The LCD is generally mounted directly on the printed circuit board using mounting bolts, screws or accessory pieces. There is often a critical alignment problem to ensure electrical coupling between the circuit board and LCD which must be taken into account when the LCD is screwed or bolted to the circuit board. LCD assembly time is thus substantial and it is difficult to subsequently disassemble the display for calibration or repair since access must be made to the fastening elements which are typically at least partially located behind a front panel. Furthermore, there is a tendency for the mounting bolts or screws to loosen in response to shock or vibration; this problem commonly occurs in hostile environments such as those to which cockpit-mounted instrumentation is exposed.
To provide an LCD assembly that is resistant to shock or vibration and further to simplify assembly and disassembly, a recently developed LCD mount (see U.S. application Ser. No. 230,409 filed on Jan. 30, 1981) comprises a holder or frame positioned between the LCD and circuit board to maintain the LCD in registration with a conductive pattern on the circuit board. Opposite sides of the holder are formed with through channels containing Zebra strips for establishing electrical connections between the conductive pattern on the circuit board and the LCD. A cover member positioned around the LCD and holder is formed with clips for snap fitting the cover to opposite sides of the circuit board. The LCD is exposed through a display opening formed in the cover. A pair of inwardly extending, resilient wings formed at the display opening are in contact with the surface of the LCD and tend to retain the LCD holder in contact with the printed circuit board; this structure retains electrical integrity between the circuit board and LCD in the presence of shock or vibration.
Because the LCD is recessed somewhat within the cover member by the resilient wings, however, the display is difficult to observe and the face of the instrument is aesthetically displeasing. The problem is aggravated when the LCD assembly is positioned behind a panel and a cover plate is located on the outer surface of the panel, causing the LCD to be even further recessed.
Accordingly, one object of the present invention is to provide a new and improved LCD assembly wherein the alphanumeric characters developed on an LCD that is recessed within a mounting assembly are optically projected to the front surface of the assembly.
Another object of the invention is to provide corrective optics for an LCD assembly to project LCD developed alphanumeric characters from a recessed LCD to the front surface of instrumentation.
Another object is to provide corrective optics for an LCD that does not add substantially to the cost of instrumentation.